1. Field of the Invention
The present invention relates to a multi-user MIMO scheduling method, and particularly to a scheduling method for allocating resource blocks generated by frequency division within a system band to an optimal user in consideration of reception quality in the frequency axis and the space axis.
2. Description of Related Art
Multi User-Multiple Input Multiple Output (hereafter, shortly referred to as MU-MIMO) is a promising communication technology for improving the cell throughput in uplink wireless access of next-generation mobile communication systems. In the MU-MIMO communication, a plurality of terminals transmit data signals with the same frequency, and a base station divides signals transmitted by a plurality of users (mobile stations or transmission devices) while considering as MIMO signals. The MU-MIMO communication system is believed to be a space division multiple access (hereafter, shortly referred to as SDMA) system using spatial channels as resources, in addition to generally used time and frequency resources. The use of SDMA makes it possible to obtain a significant multi-user diversity effect by appropriately selecting a user pair simultaneously transmitting data, and thus to improve the cell throughput of mobile communication systems.
FIG. 1 shows configuration of an uplink MU-MIMO communication system. It is assumed here that each of users' transmission devices (mobile stations or communication terminals) 301-1 to 301-M (M is a positive integer) has a single transmission antenna. However, there may be a case in which each of users' transmission devices has a plurality of transmission antennas, and data is transmitted by selecting one of the transmission antennas, or data is transmitted using the plurality of transmission antennas. When a number of simultaneously transmitting users is represented by M, each of the transmission devices 301-1 to 301-M of the users converts a data signal into an error correcting code and digital modulates the same. Each of data signals of each of the users is transmitted from each of the transmission antennas 302-1 to 302-M of the users. Reception antennas 303-1 to 303-N (N is a positive integer) receive multiplexed data signals from the users. A reception device (base station) 304 divides and demodulates the data signals from the users to decode the error correcting codes. The reception device 304 selects a user pair transmitting data simultaneously for each packet based on measurement results of channel quality of the users. A reception signal y received by the reception device 304 is represented by the following expression (3) using the following expression (1) indicating transmission symbols of the paired users J1 to JM and the expression (2) indicating a channel matrix.Sk=[SJ1 SJ2 . . . SJM]T  (1)Hk=[hJ1 hJ2 . . . hJM]  (2)y=HkSk+n  (3)In the expression (3) above, n represents a noise vector.
In case of MU-MIMO, a user pair must be selected in consideration of channel orthogonality of the user pair, and a full search method may be used for realizing optimal characteristics. In the full search method, combined MIMO capacities of all the users are computed, and a user pair whose capacity is maximal is selected. When a channel matrix in a user combination k (k=1, 2, . . . Nall) is represented by Hk, the MIMO capacity can be represented by the following expression (4).
                              C          ⁡                      (                          H              k                        )                          =                                            log              2                        ⁢                          det              ⁡                              (                                  I                  +                                                                                    P                        s                                                                    P                        n                                                              ⁢                                          H                      k                                        ⁢                                          H                      k                      H                                                                      )                                              ≈                                    M              ⁢                                                          ⁢                              log                2                            ⁢                                                P                  s                                                  P                  n                                                      +                                          log                2                            ⁢                              det                ⁡                                  (                                                            H                      k                                        ⁢                                          H                      k                      H                                                        )                                                                                        (        4        )            
In the expression (4) above, PS represents transmission power per user, and Pn represents noise power. In the full search method, a user combination kopt whose MIMO capacity C(Hk) is maximal is selected. kopt can be represented by the following expression (5).
                              k          opt                =                  arg          ⁢                                          ⁢                                    max              k                        ⁢                          C              ⁡                              (                                  H                  k                                )                                                                        (        5        )            
According to a user selection method using the full search method, computation must be done on a number of MIMO capacities C(Hk) which corresponds to the number of combinations Nall of all the users (Nall=NuCM), and thus the amount of computation becomes enormous when the number of users Nu and/or the number of simultaneously transmitting users M is great.
In order to reduce the amount of computation in the full search method, a method has been proposed in which users are selected by employing a Gram-Schmitd (hereafter, referred to as GS in abbreviation) orthogonalization to conduct sequential processing on each of multiple MIMO layers [see, for example, Z. Tu and R. S. Blum, “Multiuser diversity for a dirty paper approach,” IEEE Commun. Lett., vol. 7, no. 8, pp. 370-372, August 2003 (Non-Patent Document 1) and T. Yoo and A. Goldsmith, “On the optimality of multiantenna broadcast scheduling using zero-forcing beam forming,” IEEE J. Select. Areas Commun., vol. 24, no. 3, pp. 528-541, March 2006 (Non-Patent Document 2)]. The term “multiple MIMO layers (MIMO layers)” as used herein refers to transmitted data signals which have been independently converted into error correcting codes and modulated in MIMO multiplexing.
A description will be made on a principle of the MU-MIMO user selection method using GS orthogonalization. When QR decomposition (Hk=QkRk) is applied to a channel matrix Hk, a user combination kopt whose MIMO capacity is maximal is represented by the following expression (6).
                              k          opt                =                              arg            ⁢                                                  ⁢                                          max                k                            ⁢                                                                                      det                    ⁡                                          (                                              R                        k                                            )                                                                                        2                                              =                      arg            ⁢                                                  ⁢                                          max                k                            ⁢                                                ∏                                      m                    =                    1                                    M                                ⁢                                                                  ⁢                                  r                                      k                    ,                    mm                                    2                                                                                        (        6        )            
In the expression (6) above, rk,mm represents a diagonal element of Rk. The MIMO capacity can be maximized by selecting users such that the square of rk,mm is maximal. The GS orthogonalization is employed in order to realize this suboptimally. The GS orthogonalization corresponds to a process to perform QR decomposition while sequentially selecting users such that rk,mm is maximal.
FIG. 2 shows relationship between the GS orthogonalization and the QR decomposition. The GS orthogonalization (QR decomposition) processing ends upon selection of the M-th user from Nu users.
FIG. 3 shows a concept of the GS orthogonalization. The GS orthogonalization repeats a process in which a user whose channel vector hj can be projected as large as possible and an orthogonal axis corresponding thereto are successively selected while, at the same time, updating the projected channel vector hj(m+1) of hj projected on a complementary space Q(m)⊥ of an orthonormal system Q(m) composed of m orthogonal axes of already selected users.
FIG. 4 shows a user selection (scheduling) method using GS orthogonalization as an example of MU-MIMO scheduling methods. This scheduling method includes a channel vector measurement process 101, a channel power computation process 102, a maximum power user selection process 103, a projected channel power computation execution determination process 104, a projected channel vector update process 105, and a projected channel power computation process 106. This scheduling method computes power values (or amplitude values) of projected channel vectors of the users using the GS orthogonalization to select a maximum power user for each of the multiple MIMO layers.
The channel vector measurement process 101 measures an uplink channel vector with the use of a reference signal for each user (a sounding reference signal periodically transmitted principally when no data is transmitted). The channel power computation process 102 computes a channel power based on the channel vector of each user. The maximum power user selection process 103 selects a user whose projected channel power is maximal for each of the multiple MIMO layers. The user Jm selected for the m-th multiple MIMO layer is represented by the following expression (7).
                              J          m                =                  arg          ⁢                                          ⁢                                    max              j                        ⁢                                                                            h                  j                                      (                    m                    )                                                                              2                                                          (        7        )            
In the expression (7) above, hj(1)=hj when m=1. The projected channel power computation execution determination process 104 proceeds to the projected channel power computation process for the next multiple MIMO layer if m<M, whereas, if m=M, terminates the user selection (scheduling) and outputs user selection information.
The projected channel vector update process 105 updates, by GS orthogonalization, a projected channel vector hj(m+1) of an unselected user which is projected on a complementary space Q(m)⊥ of an orthonormal system Q(m) corresponding to a user already selected. The projected channel vector hj(m+1) is represented by the following expression (8).
                              h          j                      (                          m              +              1                        )                          =                              h            j                          (              m              )                                -                                                    (                                                      h                                          J                      m                                                                                      (                        m                        )                                            H                                                        ⁢                                      h                    j                                          (                      m                      )                                                                      )                            ⁢                              h                                  J                  m                                                  (                  m                  )                                                                                                                      h                                      J                    m                                                        (                    m                    )                                                                              2                                                          (        8        )            
The projected channel power computation process 106 computes a power of the projected channel vector hj(m+1) updated by GS orthogonalization. The projected channel power of the unselected user is input to the maximum power user selection process 103 so that a user for the next multiple MIMO layer is selected.
On the other hand, Single Carrier-Frequency Division Multiple Access (hereafter, referred to as SC-FDMA) or Orthogonal Frequency Division Multiple Access (hereafter, referred to as OFDMA) is employed for uplink wireless access in next-generation mobile communication systems. According to these FDMA methods, a wireless resource is frequency-divided into a plurality of resource blocks (hereafter, referred to as RB) within a system band. A plurality of RBs may be combined to form a carrier in order to increase the communication capacity. The frequency-divided RBs are allocated to a plurality of users. Among scheduling methods for allocating RBs to a plurality of users, a frequency scheduling method for allocating RBs to a user with the maximum priority in accordance with channel variation along the frequency axis is particularly effective in improving the throughput.
A communication system described in Japanese Laid-Open Patent Publication No. 2007-214993 (Patent Document 1) is one of known technologies for improving the throughput by manipulating the scheduling. According to Patent Document 1, a terminal-station apparatus waiting to commence communication autonomously requests a base station for spatial multiplex scheduling if a desired transmission rate can be satisfied by itself. Thus, this technique intends to improve the throughput and to reduce the load of computation of the base station by performing efficient scheduling by utilizing the request for scheduling from the terminal-station apparatus to the base station. Base stations in general are provided with a scheduler function by cooperation between software and hardware. The scheduler function is described also in Japanese Laid-Open Patent Publication No. 2007-221755 (Patent Document 2), for example. Although Patent Document 2 describes a scheduler (scheduling unit) for use in MU-MIMO, it does not mention at all a scheduling method which constitutes a principal operation of the scheduler.
FIG. 5 schematically shows frequency scheduling using a maximum CIR method. In the frequency scheduling, reception SINRs (Signal to Interference and Noise power Ratios) of users are measured for each RB so that RBs are allocated to a user with a maximum reception SINR. In the example shown in FIG. 5, RB2 and RB3 are allocated to the user #1, RB1 to the user #2, and RB4 to the user #3.
FIG. 6 shows a frequency scheduling method. This scheduling method includes a reception SINR measurement process 201 for each RB, a priority computation process 202 for each RB, a maximum priority user selection/RB allocation process 203, allocation of plural RBs 204, and determination of presence of unallocated RB on frequency axis 205.
The reception SINR measurement process 201 measures uplink reception SINRs for each RB with the use of reference signals of the users. A reception SINR is generally used as an indicator indicative of a channel quality, or CQI (Channel Quality Indicator) in mobile communication systems. Therefore, CQI may be used as the reception SINR. The priority computation process 202 computes priorities of users for each RB using the reception SINRs, based on a maximum CIR method or Proportional Fairness (PF) method. The maximum priority user selection/RB allocation process 203 selects a user with maximum priority for an unallocated RB so that the RB is allocated to the user. If the user allocated with an RB in the maximum priority user selection/RB allocation process 203 has still other RBs for which it has maximum priority, the plural RBs allocation process 204 allocates those RBs to the user. In the SC-FDMA method which requires allocation of consecutive RBs, for example, if the user has the maximum priority for RBs adjacent to the allocated RB, these RBs are allocated to the user. The frequency axis unallocated RB presence determination process 205 proceeds to scheduling for a next user if there is an unallocated RB along the frequency axis, whereas terminates the scheduling if there is no unallocated RB and outputs user selection and RB allocation information.